This invention relates to high density, high strength Si.sub.3 N.sub.4 ceramics produced by pressureless sintering of Si.sub.3 N.sub.4 powder compacts, and more particularly relates to such ceramics produced from partly amorphous, and partly crystalline Si.sub.3 N.sub.4 powder.
Si.sub.3 N.sub.4 has generated considerable interest recently as a possible substitute for superalloys in applications requiring high strength at elevated temperatures and good resistance to thermal shock and corrosion, an outstanding example of which is turbine engine components. Optimization of the physical properties of this material, particularly high temperature strength, holds out the promise of significant increases in the operating temperatures and efficiencies of turbine engines over those possible with superalloy components. Such optimization is dependent on the ability to produce bodies of high purity and high density.
Highest densities are at present obtained commercially by hot pressing powders obtained by nitridation of silicon at elevated temperatures. Such powders typically have an average particle size of about 4 microns to 10 microns and contain from 1 to several percent of impurities. See, for Example, Powder Metallurgy, 1961, number 8, page 145.
Copending U.S. patent application Ser. No. 625,330 filed Oct. 23, 1975 and assigned to the present assignee, describes a technique for producing high purity fine grain Si.sub.3 N.sub.4 powder, based upon the vapor phase reaction of a silicon halide compound with ammonia. This powder possesses a purity of at least 99.9 percent, an average particle size below one micron and typically about 0.4 microns and an adsorbed oxygen content typically less than 4 percent by weight, which may be subsequently reduced to less than 1 percent by heating the powder in dry nitrogen or other nonreactive atmosphere. In addition, the powder is characterized as being in the amorphous state, but can be crystallized e.g., by heating in a nonreactive atmosphere at a temperature in the range of from about 1550.degree. to 1600.degree. C for several hours.